The use of aqueous dispersions of photographic couplers and other hydrophobic photographically useful compounds is known in the art. Generally, dispersions of hydrophobic photographically useful materials (PUMs) in aqueous solutions are prepared by one of the following ways: milling of solid particles using the well known methods of comminution; precipitation of photographically useful materials from solution; and homogenization of a liquid organic phase containing a photographically useful material into an aqueous solution containing a hydrophilic colloid such as gelatin and, optionally, a surface active material.
Processes for homogenization of liquid organic phases frequently include the use of low boiling or at least partially water miscible auxiliary solvents, which auxiliary solvent is subsequently removed after homogenization by evaporating volatile solvent or washing water miscible solvents. Such auxiliary solvents facilitate combining couplers and/or any other hydrophobic dispersion components in a mixed solution, so that a dispersion with an oil phase of uniform composition is obtained. The solvent also lowers the viscosity of the oil solution, which allows the preparation of small-particle emulsified dispersions. The use of auxiliary solvent may also be used to form a liquid organic solution of a PUM for forming a dispersion where no permanent solvent is desired in the final dispersion. However, the use of auxiliary solvent also presents several difficulties in the preparation of photographic dispersions and elements. Auxiliary solvents can cause severe coating defects if not removed before the coating operation. Also, it is not possible, due to thermodynamic considerations, to remove 100% of the auxiliary solvent from the dispersion. This may cause other deleterious effects such as enhancing the solubility and movement of the PUM, or aid in crystallization. Further, the steps of evaporating volatile solvent from an evaporated dispersion and washing a chill-set, washed dispersion often leads to final photographic dispersions with variable concentration; so that careful analysis is necessary to determine the actual concentration of the photographically useful compound in the dispersion. Volatile or water-soluble auxiliary solvents present health, safety, and environmental hazards, with risks of exposure, fire, and contamination of air and water. The cost can be significant for the solvent itself, as can be the costs of environmental and safety controls, solvent recovery, and solvent disposal.
Alternatively, PUMs may be "directly" homogenized or dispersed into an aqueous solution in the substantial absence of any auxiliary solvent (i.e., absence of such solvents beyond trace or impurity levels). In one such direct dispersion process, the hydrophobic components desired in the dispersion, e.g., coupler and permanent coupler solvent, are simply melted at a temperature sufficient to obtain a homogeneous oil solution. This is then emulsified or dispersed in an aqueous phase, often containing gelatin and surfactant. The direct process also yields a dispersion with a known concentration of the photographically useful compound, based on the components added, with no variability due to evaporation or washing steps. No volatile or water-soluble organic solvents are needed, eliminating the hazards and costs associated with their use.
While small-particle dispersions of less than 1 micron diameter can be obtained by direct dispersion processes with appropriate emulsification conditions, the direct dispersion process in general leads to larger particle sizes than that obtained with auxiliary solvents. Additionally, for most photographic dispersions, permanent solvent is used to promote reactivity of the PUM. To compensate for the absence of auxiliary solvents in a direct dispersion process, higher levels of permanent solvent may be used. At high levels of such permanent solvents, however, the volume of the oil drops in the film will increase, thereby causing a deterioration in the optical properties of the film. Additionally, high levels of permanent solvent will also cause a deterioration in the mechanical properties of the film. Accordingly, it is desired to keep levels of permanent solvent low. However, at low levels of solvent relative to the PUM, there is typically a large increase in the liquid organic phase viscosity which makes it increasingly difficult to obtain small dispersion particle sizes for the organic phase in the aqueous solution.
Factors that affect organic phase particle size in a photographic dispersion include homogenizer power, interfacial tension, and viscosity of the water phase relative to the liquid organic phase. Increases in homogenizer power can be used to decrease dispersion particle size, but such effect is limited by process hardware. Interfacial tension can be lowered to decrease dispersion particle size by increasing the level of surface active material. However, the interfacial tension obtainable with a given surface active material levels off to a lower limit beyond the critical micelle concentration of the surfactant. It has also been found that lowering the interfacial tension has a minimal effect on reducing particle size when the oil viscosity is high. Additionally, it is not desirable to have large amounts of surface active materials, because it creates problems during coating of photographic layers of a photographic element as well as the propensity of PUMs to grow crystals during storage of dispersions.
The viscosity ratio of the aqueous phase relative to the liquid organic (oil) phase has also been found to affect dispersion particle size (see, e.g., "Encyclopedia of Emulsion Technology", Chapter II, Ed. P. Becher, Marcel Dekker, New York, 1983). Generally, as the ratio of the organic phase viscosity to the aqueous phase viscosity (at the temperature of homogenization) is decreased, smaller dispersion particle sizes are achieved. This effect is particularly evident where auxiliary solvents are used to decrease the organic phase viscosity. Increasing the homogenizing temperature during formation of a dispersion may also lower the organic/aqueous viscosity ratio. However, the use of higher temperatures is limited by the boiling point of the aqueous phase. Also, some PUMs and hydrophilic colloids, like gelatin, can chemically degrade at elevated temperatures. Another method of decreasing the organic/aqueous viscosity ratio is by increasing the level of hydrophilic colloid, which in turn increases the viscosity of the aqueous phase. While this approach helps solve the problem of particle size, it also causes the level of hydrophilic colloid, relative to the PUM, to increase. This is undesirable because photographic layer coating melts containing such dispersions will have a high level of hydrophilic colloid binder, which can limit the minimum dry thickness of films coated with such coating melts.
It is known to use synthetic polymers to increase the viscosity of aqueous gelatin solutions for coating purposes. Polymeric agents which increase the viscosity of aqueous solutions can be broadly classified into two groups: materials which have inherent viscosifying capabilities by virtue of their large molecular weight in combination with the associative nature of the polymer molecules with other like polymer molecules; and ionic polymers such as polyelectrolytes which form associative complexes with charged groups of gelatin which is the hydrophilic colloid typically used in dispersion making. It is known in the art of coating photographic materials, e.g., to incorporate polymers containing acid groups such as carboxyl, sulfonate or sulfate groups into coating solutions to increase the viscosity of coating solutions for photographic layers. U.S. Pat. No. 3,022,172, e.g., discloses sulfonates of vinyl, allyl, styrene or alkyl benzene compounds to increase the viscosity of gelatin coating solutions, in levels of 0.02-30% by weight of gelatin, to improve the uniformity of coatings. Photographic Science & Engineering Vol. 14, pages 178-183 discloses that ammonium salts of maleic anhydride and methyl vinyl ether, polystyrene sulfonate, poly vinyl ammonium phthalate, dextran sodium sulfate etc., can be employed as viscosity increasing agents for gelatin. U.S. Pat. No. 3,655,407 discloses acrylic acid/alkyl acrylate copolymers to increase the viscosity of gelatin solutions to improve coating uniformity. U.S. Pat. No. 4,166,050 and DD 213,768 disclose maleic anhydride copolymers as viscosifiers for gelatin solutions. DD 276,243 suggests the use of polymers containing mixed carboxylate sulfonate groups for viscosifying gelatin solutions and increased robustness to pH changes. DE 4,034,871 discloses copolymers of maleic anhydride having pendent sulfonic acid groups.
Polysaccharides containing anionic moieties have also been disclosed as viscosifiers for gelatin solutions. Naturally occurring polysaccharides, like carrageenan, have been disclosed in U.S. Pat. No. 3,250,620. Furthermore, synthetically modified polysaccharides containing anionic moieties have been disclosed as viscosifiers for gelatin solutions. For example, U.S. Pat. No. 3,335,128 discloses cellulose sulfate with mixed cations. U.S. Pat. No. 3,767,410 discloses polysaccharides where 50% of the hydroxyl groups are acetylated or sulfated. DE 3,914,947 discloses sulfoethyl substituted cellulose.
It is also known in the art to use surface active polymers as dispersing aids. U.S. Pat. No. 4,569,905 discloses anionic polymers which are specified to be surface active. U.S. Pat. No. 4,198,478 discloses sulfonated polymers which are also specified to be surface active. The surface active polymers act by reducing the interfacial tension at the oil/water interface. In order to be effective, they need to diffuse relatively rapidly during homogenization, therefore the preferred molecular weight of such polymers specified in these patents is less than 10,000. In order to be surface active their chemical structure also requires them to have a hydrophobic moiety on these molecules, placed within the polymer backbone or attached to the backbone, separate from the anionic moiety. Due to the presence of the hydrophobic moieties, these molecules form self-aggregates, or micelies, which make them ineffective viscosifiers even at high molecular weights. U.S. Pat. No. 4,935,338 discloses the use of anionic polymers and polysaccharides as dispersing aids for polymeric latexes. However, these dispersions are not of the oil-in-water type and are, consequently, not subject to particle size reduction.
U.S. Pat. No. 4,291,113 discloses the use of sulfonated polymers to prevent growth of particles in photographic dispersions, when they are stored at elevated temperatures. There is no suggestion, however, to use polymers of any desired molecular weight range, or at any organic phase to aqueous phase viscosity ratios within which these materials are effective at substantially reducing the particle size of the resulting dispersion.